In general, a LAN is a communication medium and means by which a relatively large number of relatively closely physically located processor-based communications devices can send and receive message communications. As an example of a LAN, approximately 250 or less devices may be located within a single office building, adjacent office buildings or surrounding area of a few miles. The medium itself usually includes electrical or optical cables, but may also include radio or open-air optical links over which signals are conducted. Each device and its interconnection to the medium is referred to as a "node". Each node of the LAN has its own unique address. Messages are communicated between nodes by including address information within the transmissions associated with each message. Transmissions are disregarded by all nodes except those to which the transmissions are addressed.
All nodes can be simultaneously addressed by "broadcast" transmissions. Each node recognizes, and may respond to, a broadcast. This behavior is typical of all LANs and will not be further mentioned, it being recognized that the following descriptions relate to network operation and characteristics for all types of transmissions.
Access to the LAN is controlled to assure that only one message is properly communicated at a time, thereby preventing two or more nodes from simultaneously initiating transmissions which might interfere with one another and with the proper operation of the LAN.
One widely used technique of access control involves a "token". A token is the term applied to a signal or pattern of signals which is addressed to a specific node and which, upon receipt at the addressed node, causes that node to have the exclusive right to initiate a message communication over the medium. The operational protocol of the LAN permits only the node which last received the token to initiate the message communication. Since the LAN typically uses only one token, there is no interference by other message communications.
The operational protocol of the LAN also causes the token to be passed among all of the nodes in a regular, complete and even rotational pattern or sequence, thereby giving each node equal access to the medium for communication purposes. Each node receives the token in its turn, regardless of whether or not that node has a message to initiate. If the node has a message to initiate, it does so after receipt of the token, and then passes the token to the next node in the rotational sequence after the message communication has been completed. If the node does not have a message to initiate, the token is immediately passed to the next node in the rotational sequence.
The even rotational sequence of token passing has the advantage of assuring each node an opportunity to initiate a message without experiencing a delay greater than a fixed upper bound or time period. The disadvantage of the even rotational token passing sequence is that a considerable amount of network time might be wasted in passing the token to nodes which have no messages to initiate.
Another type of access control is contention arbitration or Carrier-Sense Multiple Access (predominantly CSMA/CD). With contention arbitration only those nodes which have messages to communicate vie for access to the LAN medium. When a node with a message to communicate detects that no other message communication is in progress over the medium, it initiates the message communication. So long as no other node also initiates a message simultaneously, the node which is first in time has exclusive access to the LAN medium for the duration of its message. If two or more nodes initiate message communications simultaneously, a collision of the transmitted signals occurs and prevents all transmissions from being correctly delivered. The collisions necessitate subsequent re-transmissions of all such attempted message communications. Thus, in contention arbitrated LANs, each node must contend with all other nodes for access to the LAN medium.
While contention arbitration techniques allow only those nodes which have messages to communicate to gain access to the medium, inefficiencies are created when a relatively large number of nodes have messages to communicate. Generally, among a large number of contending nodes, there will be a greater amount of contention, more waiting, more re-transmissions, and less useful communication. As message traffic increases, re-transmissions can consume a considerable amount of time. Some nodes may be inherently denied even or fair access to the medium, because they will repeatedly lose in the contention. There is no upper bound to possible waiting time on a heavily loaded contention arbitrated LAN. Contention-type LANs are not pertinent to the present invention, because the present invention pertains to token based LANs. Contention-type LANs do not utilize tokens.
LANs can further be distinguished by the signal connectivity or signal transmission patterns. Bus-type LANs connect all nodes to a single common logical or electrical point, although the single common point may be physically dispersed over a large location. Transmitted signals from any node are received about simultaneously by all the other nodes and are disregarded by all the receiving nodes except the node to which the transmission is addressed. Since all signals are transmitted and received through a common connection point, the communication path between all the nodes is bi-directional, usually half-duplex.
Ring-type LANs have all of their nodes physically connected in a serial loop or ring. Each node always transmits to its neighboring node on one side in the ring and receives from its neighboring node on the other side in the ring. All the nodes in the ring serially pass the transmissions from one to another in this manner. The address information contained within the transmission causes only that node addressed to receive and utilize the information. Thus, all transmissions in a ring-type LAN circulate in an unalterable predefined unidirectional signal path through all of the network nodes around the ring. When tokens are employed in a ring-type LAN, the tokens must also follow this unalterable ring-like physical path.
Some token ring networks allow the token to be physically directed to bypass portions of the ring. The diversion is accomplished by connecting the ring through concentrators in an arrangement known as a star-wired ring. The concentrators allow the token to be passed out of the normal rotational sequence, but the usual purpose of the concentrators is to improve reliability and maintainability of the network by eliminating problem nodes from the network or by maintaining continuous signal paths to the operational nodes of the network.
Bus-type LANs have the advantage of sending signals directly from the transmitting node to the receiving node, through the common connection point. Ring-type LANs must send the signals through every intervening connected node in the ring between the transmitting node and the receiving node, which could amount to a considerable physical distance. In bus-type LANs a quiescent time period after each message transmission must be provided to accommodate signal propagation and to allow the transmitted signal and its reflections to die out, before the next message transmission occurs. Accommodation for this quiescent time somewhat delays the speed by which the transmissions and the token can be sent and hence somewhat reduces the rapidity by which the bus-type LAN can pass the token and transmit signals. A quiescent time period is not required for ring-type LANs because all communication is unidirectional, point to point between adjoining neighboring nodes, and no accommodation need be made for signals elsewhere on the network so long as the integrity of the neighbor-to-neighbor node connectivity is maintained. Consequently, transmissions and token passing can occur relatively rapidly in ring-type LANs, because there is no need for a quiescent time period. However, the physical path of signal propagation is usually longer because the signal must follow the ring between the transmitting node and the receiving node, and because each intervening node introduces a slight time delay when receiving and re-transmitting the signal.